Epsilon-caprolactam (ε-caprolactam) is the main precursor for the preparation of nylon-6. Typically, industrial production processes for ε-caprolactam are multistep and produce ammonium sulfate or other by-products. Currently, approximately 95% of the world's ε-caprolactam is produced from cyclohexanone oxime via the Beckmann rearrangement.
A precursor material for cyclohexanone oxime is cyclohexanone. Precursor materials for cyclohexanone can include cyclohexane, phenol, and benzene. Accordingly, the first step (or steps) in the production of ε-caprolactam is often a series of reductions and oxidations to form cyclohexanone from cyclohexane, phenol, or benzene. Cyclohexanone thus produced is next reacted with a hydroxylamine salt, usually the sulfate, to form cyclohexanone oxime and ammonium sulfate. The oxime is then rearranged in concentrated sulfuric acid, and the resulting lactam sulfate salt is hydrolyzed to form ε-caprolactam and additional ammonium sulfate.
An alternative route to produce ε-caprolactam is from adiponitrile via 6-aminocapronitrile (ACN). This cyclization of 6-aminocapronitrile can be conducted in the liquid or gas phase, with or without a catalyst. In this regard, U.S. Pat. No. 2,301,964 discloses a liquid phase cyclization of 6-aminocapronitrile at a temperatures of less than 380° C. and at reaction times of greater than 1 hour. U.S. Pat. No. 2,357,484, in turn, discloses a vapor phase process, having a short reaction time at temperatures between 150° C. and 500° C. Both processes use solid acid catalysts, which are prone to fouling, leading to increased operating and maintenance costs, as well as process downtime.
A. Krämer and H. Vogel., Chem. Eng. Technol. 21, 494-500 (1999) (“Krämer and Vogel”)1 discloses a continuous hydrolysis of 6-aminocapronitrile in water to produce ε-caprolactam at temperatures of between 250° C. and 350° C. and at a pressure of 250 bar. Using a feed comprising 5% by weight of 6-aminocapronitrile and the balance water, the authors demonstrated a 45% conversion of 6-aminocapronitrile and 55% selectivity towards ε-caprolactam at a residence time of 100 seconds. Other work done in the 100 to 250 second residence time range indicates that increasing residence time increases conversion of 6-aminocapronitrile. In addition, the paper teaches that selectivity significantly decreases as reaction temperature increases beyond 380° C., and the data presented implies poor yields with this approach.
Methods for increasing the conversion of amino alkane nitrile with high selectivity to the corresponding lactam continue to be sought.